Photochromism is a well known physical phenomenon that is observed with certain classes of chemical compounds. A detailed discussion of this phenomenon can be found in “Photochromism: Molecules and Systems”, Studies in Organic Chemistry 40, edited by H. Durr and H. Bouas-Laurent, Elsevier, 1990.
A number of substituted 3H-naphtho[2,1-b]pyrans are known to be capable of exerting a reversible photochromic effect as described for example in WO 99/31082, U.S. Pat. No. 6,630,597, U.S. Pat. No. 5,552,090, U.S. Pat. No. 5,520,853, and U.S. Pat. No. 5,623,005. However, none of these 3H-naphtho[2,1-b]pyrans compounds are reported to have dichroic properties.
Passive photochromic devices, i.e. devices containing photochromic dyes whose absorbance depends only from the presence or absence of UV light, typically exhibit rather quick activation (coloration) but it generally takes several minutes or even tens of minutes to revert from the coloured to the bleached state. This slow fading is a severe drawback for the user of photochromic glasses who has to take them off to have clear vision when leaving the sunlight and entering dimmer light conditions.
Therefore, there is a need for photochromic dyes exhibiting not only good photochromic properties, such as high absorption in the coloured state, fast colouring and fading rates, but which also may be capable of dichroism and linear light polarization when in a spatially ordered condition, for example when incorporated into liquid crystals or oriented polymer host materials.